1. Field of the Invention
The present invention relates to a process for cross-linking liquid organopolysiloxane compositions obtained from (i) a polysiloxane comprising hydroxysilyl groups containing no Si-H bonds and no organic radicals which include sites of activated ethylenic unsaturation, and from (ii) an aminopolyalkoxysilane. More especially according to the invention, the cross-linking process is carried out by simply exposing the compositions to ultraviolet radiation. Depending upon the choice of the various constituents of the composition, the subject process enables obtainment, in particular, of good non-stick coatings on various cellulosic materials, and also protective coatings, which can be used, for example, for coating printed circuits.
2 . Description of the Prior Art
Numerous processes are well known to this art for hardening or curing organopolysiloxane compositions obtained from a polysiloxane comprising hydroxysilyl groups and from an aminopolyalkoxysilane and/or a polyalkoxysilane.
Such processes can be carried out by employing catalysts which ensure rapid cross-linking on exposure to moisture, if appropriate after heating to moderate temperatures; the catalysts are typically selected from among amines, organo-tin compounds, alkyl titanates, titanium chelates or carboxylates, and the like.
The subject compositions can also be cross-linked by exposing a thin layer of the latter to various radiations. In particular, it is known that it is possible to photocross-link certain organopolysiloxane compositions obtained from an organopolysiloxane comprising hydroxysilyl groups and from an aminoalkoxysilane bearing activated unsaturated groups (acroyloxy, methacroyloxy or the like). The activated groups are those which undergo polyaddition under the effects of ultraviolet radiation; alkenyl or cycloalkenyl radicals, such as vinyl, allyl or cyclohexenyl groups, do not photopolymerize and obviously are not therefore considered as radicals including sites of "activated ethylenic unsaturation". Photocross-linkable compositions such as described above are disclosed, for example, in Japanese patent Nos. J5 4038-393 and J5 4063-200.
It too is known that UV radiation can be used to cross-link organopolysiloxane compositions containing no hydroxysilyl groups, but containing activated unsaturated groups [compare, inter alia, French patent No. 2,295,959]or, alternatively, having Si-alkenyl groups and Si-H groups (French patent No. 2,245,729).
By virtue of its manifest simplicity, the photocross-linking technique is particularly valuable in industry. However, this method, which makes it possible to easily provide, for example, a very thin non-stick coating onto a sheet of paper, or alternatively a protective coating onto printed circuits, is nevertheless restricted at the present time to the use of but certain organopolysiloxane compositions such as defined above, containing organic groups including sites of ethylenic unsaturation, generally activated sites of ethylenic unsaturation, and, if appropriate, Si-H bonds and/or hydroxysilyl groups.
Therefore, serious need exists in the photocross-linking art for yet additional genera of organopolysiloxane compositions obtained from a polysiloxane comprising hydroxysilyl groups which are also adapted for facile photocross-linking/curing.